KB,
I use a lot of what they call 'spinning' or 'casting', and even some 'ice rod' blanks to build fly rods on. (Once in a while I'll even use a blank marked 'fly' if I can't find a better option ).
The blank itself has no idea what kind of rod it'll be use for.
As long as you put the proper line on it, ANY blank can cast a fly line.
And, if you really want a 'fast action' fly rod, you HAVE to use a spinning blank...while they may 'claim' to make 'fast action' fly rods, they are only 'fast' compared to fly rod blanks...
Buddy
It Just Doesn't Matter....